Long-term objective: To reduce the epidemic of child obesity and related disorders, which is partly due to youths being exposed to "obesogenic" environments that favor sedentary activities (TV watching, video games) and energy-dense snacks, while discouraging moderate-vigorous physical activity (MVPA). Specific aims: To test the hypotheses that (1) youths attending schools that provide a "fitogenic" after-school environment that encourages MVPA and healthy snacks will increase less in percent body fat (%BF) over a 3-yr period than youths who do not attend such schools; and (2) the fitogenic environment will lead to favorable changes in cardiovascular (CV) fitness, CV disease risk profile and psychosocial characteristics. Formative research: Two schools will be randomly assigned to control and experimental conditions for a pilot study to guide refinement of techniques to be used in the main trial; the pilot intervention will last for 3-mo. Design of main trial: 18 schools in Richmond County, Georgia, which is ca. 63% African-American, will be randomly assigned to control or experimental treatments; school (rather than individual) will be the unit of randomization and analysis. Measurements taken at the beginning and end of each of three grades (3rd-5th) will include: %BF with dual-energy x-ray absorptiometry; CV fitness with a modified Harvard Step Test; blood pressure, the total cholesterol to high density lipoprotein cholesterol ratio; anthropometry (height, weight, waist circumference); psychosocial variables related to sport and PA; and free-living PA and diet. All schools will receive information about healthy eating and PA. All 3rd graders in the control schools will be invited to participate in the testing alone, while all 3rd graders in the experimental schools will be invited to participate in both the testing and the after-school sessions, which will last 120 min each school-day afternoon. For 40 min, healthy snacks and homework assistance will be provided. The 80 min of MVPA will emphasize developmentally appropriate games and dance forms, offered in a climate conducive to development of sport competence, enjoyment and skills. The hypotheses will be tested with treatments x time analyses of variance to see if the changes in %BF and associated variables over the 3-yr period are more favorable in the experimental than in the control group. Potential public health value: If the data support the effectiveness of the intervention, then communities may be encouraged to foster fitogenic after-school environments to prevent obesity and associated health problems.